Conventional automatic vehicles may include a transmission having a torque converter to provide a hydrodynamic coupling with torque multiplication. The hydrodynamic coupling allows the engine to continue running while connected to the transmission when the vehicle is stationary. In addition, the torque converter provides torque multiplication to assist vehicle launch and provides damping of driveline torque disturbances. The torque multiplication or torque ratio varies with the speed difference or slip between the torque converter input element (impeller) and output element (turbine). A torque converter clutch or bypass clutch may be provided to mechanically or frictionally couple the impeller and the turbine to eliminate the slip and associated losses to improve efficiency. However, driveline torque disturbances are then more easily transmitted to the vehicle cabin and may result in noise, vibration, and harshness (NVH) and reduce vehicle driveability. As such, the torque converter bypass clutch is usually disengaged or released when the vehicle operating conditions are likely to produce driveline torque disturbances.
Various hybrid vehicle configurations have been developed that utilize both an engine and a motor to drive a vehicle through a transmission, which may be implemented by various types of transmissions that may or may not include a torque converter depending on the particular application. For example, a continuously variable transmission (CVT) or automated manual transmission (AMT) may not include a torque converter whereas a step-ratio automatic transmission having a torque converter may be used to provide similar advantages as in a conventional powertrain as previously described.
Hybrid vehicles generally include an electrical drive mode where the motor is used to power the vehicle and the engine is off. Applications having a torque converter bypass clutch may engage or lock the bypass clutch in the electrical drive mode to improve efficiency. Another hybrid vehicle operation mode uses both the engine and the motor to power the vehicle. A rolling engine start may be used when the vehicle is moving to transition from the electrical drive mode to the hybrid drive mode. The bypass clutch is typically disengaged during engine start to mitigate associated driveline torque disturbances. However, this reduces efficiency as previously described. As a rolling engine start event happens, the traditional control approach does not address the complexity of the power transition and its impact on the driveability.